Ballasting circuitry for discharge lamps is known in the prior art. A basic lamp ballasting circuit has several functions:
1. To preheat the lamp electrodes so that they will emit electrons. PA1 2. To provide sufficient open circuit voltage to initiate an arc. PA1 3. To limit the current flowing through the negative resistance arc. PA1 4. To reinitiate the arc on each half cycle of the applied AC voltage. PA1 5. To regulate the lamp current against line voltage changes. PA1 6. To minimize power loss. PA1 7. To provide a high power factor. PA1 1. To provide low harmonics to the lamp in order to extend lamp life. PA1 2. To reflect low harmonics to the power line. PA1 3. To reflect a low ripple to the power line. PA1 4. To operate the lamp at a frequency significantly higher than the power line frequency in order to reduce lamp flickering and to increase luminous efficacy. PA1 5. To reduce electromagnetic interference (EMI) and radio frequency interference (RFI).
These competing design goals have encouraged the design of a large number of lamp ballasting circuits. The conventional solution was to use large reactors to limit arc current without incurring resistive losses, to provide regulation, and to produce high voltages for igniting the arc. However, the conventional circuits suffered from high losses in the inductor cores.
Developments in power electronics have encouraged design efforts to further reduce losses and to extend lamp life by improving power conditioning. The electronic ballast further design goals include:
The prior art includes many different electronic ballasting circuits. The conventional electronic ballast comprises an AC to DC rectification first stage followed by a DC to AC inversion second stage. Early examples of this circuit are U.S. Pat. No. 4,463,286 (Justice) and U.S. Pat. No. 4,469,988 (Cronin). The '286 and '988 patents teach simple open-loop circuits that provide high frequency sinusoidal signals to pre-heat, start and run discharge lights.
However, the simpler conventional electronic ballasts such as the '286 and '988 patents have limitations. One limitation is the poor regulation these circuits provide. Because of the open-loop nature of the circuits, lamp intensity varies with fluctuations in the power line. Further, there can be difficulties in maintaining the stability of the inverter oscillations under certain conditions such as lamp burnout. Instability in a circuit that switches power signals can also be a significant problem.
Attempts have been made to overcome these limitations of the conventional electronic ballast. U.S. Pat. Nos. 4,983,887 and 5,039,919 (both continuations-in-part of U.S. Pat. No. 4,819,146) to Nilssen describe an intricate circuit and a complicated feedback control system. The '919 patent teaches a circuit that employs feedback from: a) the magnitude of the inverter's output current, b) the magnitude of the voltage present across the tank-capacitor, c) the magnitude of the current flowing through the lamps, d) the setting of an adjustment means operative to adjust the amount of light output, e) a temperature associated with the inverter, and f) the magnitude of any ground-fault current that might be flowing.
The circuit of the '887 patent is intricate and still suffers from essentially the same problems as the prior conventional electronic ballasts. The difficulty in regulating and stabilizing the inverter stage of the conventional circuit means that the circuit parameters have to be continuously adjusted with the effect that the circuit does not run cleanly or efficiently. With the large number of discharge lamps in existence, it is desirable that electronic ballast perform efficiently and interfere minimally with the surrounding electrical environment, including power lines.
An improved electronic ballast is taught in U.S. Pat. No. 4,870,327 (Jorgensen). The '327 patent teaches a structure for an electronic ballast which comprises an AC to DC rectification first stage followed by a DC to DC conversion second stage followed by a DC to AC inversion third stage and a control stage for both the converter and inverter which accepts feedback from both the converter and inverter.
The circuit of the '327 patent also has drawbacks. The main drawback arises from the complexity of the control and feedback circuitry. It uses either one or two pulse width modulation integrated circuits to analyze the inverter input voltage, the inverter input current and the converter peak current to continuously set the pulse width of the converter and the frequency of the inverter.